1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a demand limit circuit for enhancing power management during spin down.
2. Description of the Prior Art
When the disk spins down in a disk drive it is important to park the head before the air bearing dissipates to prevent damage to the head and/or the disk. In disk drives wherein the head is parked in a landing zone on the disk, it is also important to brake the spindle motor as quickly as possible to minimize head wear. Conventionally, during a power failure the head is parked using the back EMF (BEMF) voltage present over the motor windings due to the angular momentum of the spindle and disk assembly. The current induced by the BEMF charges a capacitor to thereby generate an internal supply voltage which is applied to a voice coil motor (VCM) to park the head. Once the head is parked, a braking torque is applied to the spindle motor to stop it from rotating as quickly as possible in order to minimize head wear.
If the internal supply voltage drops below a certain threshold for an extended period of time during a power failure mode, the analog and digital circuitry which control the power down sequence may malfunction or shut down. This can typically occur if the VCM driver draws an excessive amount of current from the internal supply voltage. For example, if the head is in the middle of a seek operation when power failure occurs, the VCM driver may draw excessive current in order to decelerate the head. In another example, the VCM driver may draw excessive current in order to prevent the head from bouncing away from the parking latch at the end of the park operation. Either of these events may pull down the internal supply voltage beyond a safe level, thereby rendering the power down operation questionable.
This problem has been addressed in the prior art by designing disk drives with efficient spindle motors capable of providing sufficient BEMF voltage during power failure, and with efficient VCM motors which require less current to perform the head parking operation. However, this increases the overall cost of the disk drive since less efficient spindle and VCM motors are less expensive. Another known technique which alleviates this problem is to employ a xe2x80x9cboostxe2x80x9d circuit for xe2x80x9cboostingxe2x80x9d the internal supply voltage by periodically shorting the spindle motor windings. For example, U.S. Pat. No. 5,504,402 discloses a boost circuit for boosting the internal supply voltage by periodically grounding the spindle motor windings using a grounding switch. When the grounding switch is turned on (grounded), a current builds in the spindle motor windings due to the inductance and the BEMF. When the current reaches a predetermined level, the switch is turned off so that the current stored in the spindle motor windings charges a capacitor which boosts (and filters) the internal supply voltage. When the internal supply voltage reaches a predetermined level, the grounding switch is turned back on in order to recharge the current in the spindle motor windings. Although the boost circuit increases the available power so that less efficient spindle and VCM motors may be employed, further improvements are attainable.
Another problem identified with prior art disk drives is the inability to safely park the head unless the disk is rotating fast enough so that sufficient power is available to retract the head. Thus, the head is typically positioned over the landing zone on the disk while the disk is still rotating at a high RPM resulting in undesirable head wear. The landing zone is typically textured to reduce the stiction force during spin up; however, this textured surface also wears on the head during spin down. Because prior art techniques position the head over the landing zone while the disk is still spinning at a high RPM, head wear increases due to the increased time to brake the spindle motor. The prior art boost circuit alleviates this problem somewhat by increasing the internal supply voltage, thereby enabling head parking at a lower RPM which reduces the braking time while the head is over the landing zone. However, further improvements are attainable.
There is, therefore, a need to improve upon prior art techniques for generating an internal supply voltage used to park the head in a disk drive during spin down. In particular, there is a need to protect against the internal supply voltage from dropping below a safe level in order to maintain proper operation of the digital and analog circuitry responsible for the power down sequence. Further, there is a need to improve power management so that head parking is viable at a lower RPM to reduce head wear.
The present invention may be regarded as a disk drive comprising a disk, a head, a voice coil motor (VCM) for actuating the head radially over the disk (the VCM having a voltage control input), and a spindle motor for rotating the disk, the spindle motor comprising a plurality of windings and a rotor rotatable at a variable spin rate wherein the rotor generates a back EMF (BEMF) voltage across the windings proportional to the spin rate of the rotor. A plurality of switching elements are coupled to the windings, and switch control logic generates switch control signals applied to the switching elements for commutating the spindle motor during normal operation and for generating an internal supply voltage Vi from the BEMF during a spin down mode, the internal supply voltage Vi applied to the voltage control input of the VCM for parking the head. A demand limit circuit disconnects the internal supply voltage Vi from the voltage control input of the VCM when the internal supply voltage Vi falls below a predetermined level, and reconnects the internal supply voltage Vi to the voltage control input of the VCM once the internal supply voltage Vi recharges to a predetermined level, thereby enhancing power management during the spin down mode.
The present invention may also be regarded as a method of enhancing power management in a disk drive during a spin down mode, the disk drive comprising a disk, a head, a voice coil motor (VCM) for actuating the head radially over the disk (the VCM having a voltage control input), and a spindle motor for rotating the disk, the spindle motor comprising a plurality of windings and a rotor rotatable at a variable spin rate wherein the rotor generates a back EMF (BEMF) voltage across the windings proportional to the spin rate of the rotor. A plurality of switching elements are coupled to the windings, and switch control logic generates switch control signals applied to the switching elements for commutating the spindle motor during normal operation and for generating an internal supply voltage Vi from the BEMF during a spin down mode, the internal supply voltage Vi applied to the voltage control input of the VCM for parking the head. The method of enhancing power management during the spin down mode comprises the steps of disconnecting the internal supply voltage Vi from the voltage control input of the VCM when the internal supply voltage Vi falls below a predetermined level, and reconnecting the internal supply voltage Vi to the voltage control input of the VCM once the internal supply voltage Vi recharges to a predetermined level.